A superconducting magnet is an electromagnet made from coils of superconducting wire, tape or cable which operates in a superconducting state to conduct much larger electric currents than is possible with ordinary wire, thereby creating intense magnetic fields. Superconducting magnets are commonly used in MRI (magnetic resonance imaging) machines and in NMR (nuclear magnetic resonance) spectrometers.
Superconducting magnets currently known in the art may include electrical insulation positioned between the turns of the superconducting coil. With the electrical insulation positioned between the turns of the superconducting coil, the electric current in the magnet is only permitted to flow in a direction parallel to the length of the superconductor. In general, the current flowing in a direction along the length of the superconductor is herein referred to as a parallel current and the current flowing in a direction transverse to the length of the superconductor, across turns of the superconducting coil, is herein referred to as a transverse, or radial, current. In addition, the current flowing from the positive lead to the negative lead of the superconducting coil is herein referred to as the superconducting coil current, wherein the superconducting coil current is equal to the sum of the net parallel current and the net transverse current within the superconducting coil.
Voltage differentials in superconducting coils are a result of either resistive current flow, as described above, or a result of the inductance of the superconducting magnet itself. Resistive portions and the associated resistive current flow and resistive voltage differentials are created in the superconducting magnet as a result of temperature increases from discrete or global heat sources such as wire motion, epoxy cracking, insufficient cooling, flux jumps, radiation or other sources. Resistive current flow may also result from the operation of the superconducting magnet at magnetic fields, currents, temperatures, and/or strains which are above one or more critical limits of the superconducting material. Additionally, the resistive current flow may vary along a length of the superconducting material. Voltages due to resistive current flow in a resistive portion of the superconducting material are herein referred to as resistive voltages. Resistive voltages within the superconducting magnet produce internal heating within the superconducting magnet, in the region of the resistive voltage, wherein the internal heating is equal to the product of the resistive voltage and the current flowing from the higher voltage potential (VU) to the lower voltage potential (VL). Internal heating of the superconducting magnet due to resistive voltages is herein referred to as resistive heating.
Inductive voltages in the superconducting magnet are created by magnetic field or electric current changes on, or in, the superconducting magnet as a result of the inductance of the superconducting magnet. Voltages due to inductance of the superconducting magnet are herein referred to as inductive voltages. Inductive voltages in superconducting magnets having electrical insulation between the coils do not result in resistive heating. However, resistive voltages in superconducting magnets having electrical insulation between the coils do result in resistive heating, wherein the resistive heating is equal to the product of the superconducting coil current and the resistive voltage.
In the case of superconducting magnets having metallic or electrically conductive layers between the coils, inductive voltages do not generate resistive heating as a result of the parallel current. However, resistive heating is generated as a result of the transverse current in superconducting magnets having metallic or electrically conductive layers between the coils, wherein the resistive heating is equal to the product of the transverse current of the superconducting current and the transverse voltage between turns of the coil. In addition, resistive voltages in superconducting magnets having metallic or electrically conductive layers between the coils generate resistive heating as a result of the parallel current and the transverse current, wherein the resistive heating is equal to the sum of the product of the longitudinal resistive voltage and the parallel current and the product of the transverse resistive voltage and the transverse current.
In any superconducting coil, resistive heating tends to increase the temperature of the material in the resistive portion of the superconducting coil, and in the local proximity of the resistive portion of the superconducting coil as the temperature gradients conduct heat away from the resistive portion. If protective action is not taken to prevent this resistive heating, the peak temperatures and temperature gradients across the coil can permanently damage the superconducting coil.
In a superconducting magnet employing a metallic or electrically conductive layer between the turns of the coil, the requirement for detection of the resistive state and activation of a protection scheme is reduced or eliminated as a result of the presence of the alternate current path that is transverse to the length of the superconductor. The allowed transverse current is effective in distributing the resistive heating of the superconducting coil over a larger volume and thereby provides superior protection from resistive voltages, as compared to the superconducting coil having electrical insulation between the coils. The reduction or elimination of resistive voltage detection and protection schemes is a desirable characteristic because it reduces the costs associated with implementing the detection and protection schemes. In addition, improved protection from resistive voltages provided by the metallic or electrically conductive layers in the coil reduces the risk of permanently damaging the superconducting coil. However, in the superconducting magnet having metallic or electrically conductive layers, the operational inductive voltages of the superconducting material produce undesirable resistive heating as a result of the transverse current flow across turns of the coil. Resistive heating raises the temperature of the superconducting coil which requires more refrigeration capacity or longer recovery times to return to operational temperatures, both of which are costly and therefore undesirable in a superconducting magnet. In addition to the undesirable resistive heating from the transverse current, the transverse current does not contribute to the desired produced magnetic field of the superconducting magnet. The transverse portion of the superconducting coil current has a decay time constant equal to the ratio of the coil inductance divided by the resistance to transverse current flow. As such, the desired produced magnetic field does not change linearly in time along with the changing coil current because a portion of the coil current is directed transversely to the superconductor length and, as such, does not contribute to the desired produced magnetic field. The non-linear magnetic field versus superconducting current is an undesirable performance characteristic of a superconducting magnet having a metallic or other electrically conductive layer or coating.
In addition to superconductors having electrical insulation between turns of the coil, superconducting coils are also known in the art that do not include electrical insulation between the turns of the coil, or alternatively, superconducting coils are known that include a metallic layer or an other electrically conductive layer between the turns of the superconductor. In a superconducting magnet without an electrical insulation between the coils, or with an electrically conductive material positioned between the coils, electric current can flow both parallel and transverse to the length of the superconducting material. However, in a superconducting magnet having an electrical insulation between the turns of the coil, the electric current is only permitted to flow parallel to the length of the superconducting material. In a superconducting magnet having an electrical insulation layer between the turns of the coil, detection of the resistive state in the superconducting coil and activation of a protection scheme to more evenly internally distribute the magnetically stored energy and resistive current from a power supply throughout the superconducting coil and/or to distribute the stored energy and resistive current to hardware external to the superconducting coil to prevent damage, may be installed to safely distribute the energy over as large a volume of the coil as possible, thereby preventing peak temperatures and temperature gradients from damaging the magnet. In addition, further protection of the superconducting coil in the resistive state may be provided by employing a resistive material, such as copper, within the superconducting coil itself or in a metallic conductor wound together with the superconducting coil. While the metallic conductor is effective in decreasing the overall current density of the superconducting coil in a resistive state to protect the superconducting coil from damage, incorporating a metallic conductor into the superconducting coil also reduces the efficiency of the superconducting coil in producing the desired magnetic field, which is undesirable.
Accordingly, what is needed in the art is a superconducting magnet which provides a barrier to current flow transverse to the length of the superconductor, while also providing a magnetic quench protection mechanism. However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the field of this disclosure how the shortcomings of the prior art could be overcome.